1. Field of the Invention
The present invention is directed to an apparatus and method for making grayscale photo masks used as a light filter to transfer an image from one device to another and optical elements made using such masks.
2. Discussion of the Related Art
Conventional photolithographic technology is based on binary photolithography, which basically produces a step-like binary structure on a material of interest. Grayscale technology is a type of photolithographic technology that is mainly used in the opto-electronics industry. Grayscale photolithography has been developed to make multi-level steps or ultimately a continuous structure.
FIG. 16 illustrates a conventional binary photolithographic process, and FIG. 19 shows an example of a binary diffraction grating made with a binary photo mask.
As illustrated in FIG. 16, a type of conventional binary photo mask is made by a glass wafer with a chromium pattern on the wafer, where the transmission is binary, i.e., 0 or 1. In FIG. 16, step 1 involves preparation of a wafer (for example, glass) on the surface of which a micro-structure is to be formed. In step 2, the wafer is placed on a vacuum chuck, and then, using a spin-coater, a photo resist is spin-coated onto the wafer, by dropping resist drops onto the spinning wafer. Step 3 represents preparation of the photo mask. An exposure step (step 4) is performed using a mask aligner or a stepper by exposing the photo resist with light of a certain wavelength through a photo mask loaded on an optical system to project light onto the photo resist on the wafer to make a desired pattern on the photo resist corresponding to the pattern formed on the photo mask in advance. A development step (step 5) is performed to develop the photo resist on the wafer, where the exposed portion is removed and the unexposed portion stays on the wafer. Next, an etching step (step 6) is performed to etch the wafer with developed photo resist using any plasma etcher or chemical etcher, where the remaining photo resist plays a role to avoid being exposed in the plasma or chemical etcher while bare parts of the wafer are etched to make a structure corresponding to the original pattern on the photo mask.
An example of a photolithographic process is described in European Patent Application 0 588 781 which discloses a method for exposure of a pattern by writing with a focused laser light onto a photosensitive substrate. During the writing procedure, the substrate is moved in a first direction, and the optics are continuously moved to focus laser light on the photosensitive surface in a second direction orthogonal to the first direction. The focused laser light is spread on the photosensitive surface in the first direction to form an extended focus area, and the focused laser light is independently controlled to hit position increments spaced in the first direction across the extended focus area. Another example is described in U.S. Pat. No. 6,490,390 which discloses an optical interferometric system for fabricating fiber Bragg gratings in photosensitive optical fibers by using an acousto-optic element to generate and control the radiation pattern with a high resolution. The radiation pattern may be programmable in that different radiation patterns may be produced by controlling the acousto-optic element. The fiber is exposed to an interference pattern to write a grating pattern in the fiber.
A gray scale photo mask is a mask that has a desired pattern provided on a given material described by the transmission of light and is used to project light onto a massive number of wafers having a photo resist thereon. A grayscale photo mask may be made using any number of materials using a technique to alter the transmission of the material to be multi-level or gray-level.
FIG. 18 shows a conventional grayscale photolithographic process to make a grayscale photo mask, using LDW (Laser Direct Write) glass. LDW glass is a kind of grayscale photo mask material made by Canyon Materials Inc., San Diego, Calif. Such materials and their use are described in U.S. Pat. No. 6,524,756, U.S. Pat. No. 6,562,523 and Korolkov et al., “Application of gray-scale LDW-glass masks for fabrication of high-efficiency DOEs,” Institute of Automation and Electrometry SB RAS, Novosibirsk, 630090, Russia, January, 1999.
The base transmission of LDW glass is set at a desired optical density (OD) at a certain wavelength, normally OD=2.0, for example, which means incident light transmits only approximately 1% through the glass. The transmission of LDW glass illuminated by a laser changes in accordance with the laser power. As the laser power increases, LDW glass turns more transparent. By traversing the surface of the LDW blank while modulating the laser power in accordance with desired patterns, a grayscale photo mask is obtained whose transmission varies across the surface of the LDW glass corresponding to a desired pattern. In this way, a grayscale optical density, or alternatively grayscale transmission, can be attained. This allows a grayscale structure to be made on the photo resist on a wafer to make a grayscale optical element by final etching. FIG. 17 shows the relation between transmittance and laser power of an LDW material.
More particularly, in order to make a grayscale photo mask using LDW glass, a focused laser beam is required. The laser should have a certain power to bleach LDW glass that has a threshold energy to be bleached within a practical writing time. Also, the laser should have sufficient focusing power to write a very fine pattern, e.g., having a beam spot of 1 micron or less. An Acousto-Optic Modulator (AOM) modulates the laser beam to vary the intensity of the transmitted laser beam applied to the LDW glass surface. The laser writer for writing on the LDW glass includes a laser, an AOM, a scanning mirror, a focusing lens, a translation stage, and a precise positioning mechanism using, for example, an interferometer and a computer. An example of a laser writer is that made by Microtech (Italy) having a 150 mW He—Cd laser with a 442 nm wavelength.
With respect to the writing method, to write up a pattern on the LDW blank, a raster scanning method may be used. The scanning speed depends on how fast it is desired to write the pattern, for example, a 39 cm/s scan using an objective lens of NA=0.65 (NA shows focusing power) and a gas laser of 150 mW power to make a grayscale pattern with a full range of OD=2.0.